The present invention relates generally to a technique in digital mobile communications and, more particularly, to a technique for canceling noise in a microphone communications path.
In a digital mobile phone, communications are conducted through two possible communications paths. In the first communications path, a microphone of the mobile phone picks up the voice activity of a human user, the subsequent voice activity is converted to an electrical signal, the electrical signal is converted by an analog-to-digital converter into a digitized information stream, the digitized information stream is modulated onto a radio carrier, and the modulated radio carrier is then transmitted over a radio link to a receiver of a base station. In the second communications path, the base station transmits a radio carrier modulated by digital information to the mobile phone, the modulated radio carrier is demodulated by a demodulator of the mobile phone, the demodulated waveform is passed to a digital-to-analog converter, and the analog output of the digital-to-analog converter is directed to a loudspeaker.
A mobile phone implementing the above communications paths comprises many discrete physical components packed into a small area. Consequently, electromagnetic energy of a particular frequency may escape from some of these components into the surrounding environment potentially causing noise interference to the other components of the mobile phone. Of particular concern to a designer of a mobile phone is the microphone and loudspeaker of the mobile phone, both of which are subject to picking up this noise from the other components of the mobile phone. This is because the wire connecting the microphone to the analog-to-digital converter and the wire connecting the digital-to-analog converter to the loud speaker are both potentially vulnerable to picking up any electromagnetic energy transmitted from any of the other components. A particular problem is the 217 Hz sending frequency radiated by A Time Division Multiple Access (TDMA) transmitter of a GSM mobile phone. This noise when heard by human ears resembles the sound of a bumblebee and is thus known as bumblebee noise.
Previously, the problem of noise from other components has been solved by careful design of the wires to the loudspeaker and from the microphone. However, this is not an efficient solution to the problem of electromagnetic interference because this solution requires an experimental arrangement of physical components by a skilled designer.
In view of the foregoing, it would be desirable to provide a technique for canceling noise (such as bumblebee noise) in microphones which overcomes the above-described inadequacies and shortcomings. More particularly, it would be desirable to provide a technique for canceling noise in microphones in an efficient and cost effective manner.
According to the present invention, a technique is provided for canceling noise in a microphone communications path. The microphone converts speech to a voice signal. An electrical equivalence circuit is placed in close proximity to and electrically matches the microphone so as to produce a signal free reference signal. An analog multiplexer alternately switches between the microphone to the electrical equivalence circuit to produce a multiplexed signal comprising the electrical voice signal from the microphone and the signal free reference signal from the electrical equivalence circuit. A communications path (typically a wire) connects the analog multiplexer to an A/D converter. The communications path carries the multiplexed signal through a noise intensive environment such that the multiplexed signal acquires a noise component. The A/D converter converts the multiplexed signal having the noise component to a plurality of voice samples and a plurality of noise samples. A noise cancellation unit applies a noise suppression procedure (e.g., spectral subtraction) to the plurality of noise samples and the plurality of voice samples to reproduce the voice signal without the noise component.
In a further aspect of the present invention, the plurality of noise samples are taken from the signal free reference signal of the multiplexed signal having the acquired noise component.
In yet a further aspect of the present invention, the plurality of signal samples are taken from the voice signal of the multiplexed signal having the acquired noise component.
In another aspect of the present invention, the analog multiplexer switches from the voice signal to the signal free reference signal at a rate of 16 kHz.
In still another aspect of the present invention, the A/D converter samples the multiplexed signal at a rate of 16 kHz.
In another aspect of the present invention, the voice signal from the microphone is sampled by the A/D converter at an 8 kHz rate.
In yet another aspect of the present invention, the signal free reference signal is sampled by the A/D converter at an 8 kHz rate.
In still another aspect of the present invention, the noise component includes bumblebee noise centered at approximately a 217 Hz signal.
In yet another aspect of the present invention, the noise cancellation unit applies a spectral subtraction procedure to the plurality of noise samples and the plurality of voice samples to produce a voice signal without the noise component.
In still another aspect of the present invention, there is a transmitter and the noise component is a result of electromagnetic energy generated by the transmitter radiating the electromagnetic energy centered at a predetermined frequency. Typically, the predetermined frequency is approximately 217 kHz.
In still another aspect of the present invention, a microphone converts speech to a voice signal. An electrical equivalence circuit, in close proximity to and electrically matching the microphone produces a signal free reference signal. A first communications path carries the voice signal from the microphone to the analog multiplexer. A second communications path carries the signal free reference signal from the electrical equivalence circuit to the analog multiplexer. The first and second communications paths are carried through a noise intensive environment such that the voice signal and the signal free reference signal both acquire a noise component. An analog multiplexer, connected to the first and second communications paths, alternately switches between the microphone and the electrical equivalence circuit to produce a multiplexed signal comprising the voice-laden component voice signal and the noise-laden component signal free reference signal. An A/D converter coupled to the analog multiplexer converts the multiplexed signal having the noise component to a plurality of voice samples and a plurality of noise samples. A noise cancellation unit applies a noise suppression procedure to the plurality of noise samples and the plurality of voice samples to reproduce the voice signal with the noise component suppressed.